248 



FUNDAMENTALS OF CYTOLOGY 



From the point of view of fitness to the environment it is evident that the 

 ecologically important unit is not the species, but the regional climatic race, or, 

 to adopt Turesson's term, the ecotype. Several of these may combine to form a 

 species, or a single ecotype may develop an isolating genetic barrier to form a 

 monotypic species. . . . Such monotypic species occui)y a narrow climatic 

 belt and show little variation and adaptability. However, it makes little 

 difference whether a given area is populated by a series of ecotypes belonging to 

 one species, or by a series of monotypic species belonging to one species complex, 



Fig. 176. — Diagram representing relationships in the genus Layia as indicated by the 

 combined results of taxonomic, ecological, and cytological studies. Circles represent species 

 with the chromosome numbers shown; shaded connections show degree of genetic affinity; 

 width of solid black lines represents degree of chromosome pairing in interspecific hybrids. 

 The dotted lines indicate major morphological breaks in the genus. {After J. Clausen, 

 D. D. Keck, and W. M. Hiesey.) 



or by a combination of both. The evolutionary past and future differ, however, 

 in the three instances. 



Evolutionary processes have left plants arranged in groups of various order 

 and separation, such as populations, ecotypes, species and species complexes. 

 These groups indicate stages in evolutionary differentiation, and they have 

 evolved only where there is a diversity of environments. 



There are many mechanisms by which living things can increase their heredi- 

 tary variation, but regional differentiation requires the discriminating selection 

 offered by unlike environments. We have no evidence that the direct influence 

 of environment produces fundamental hereditary changes in species, but major 

 alterations in environments provide new habitats and refuges for the products 



